The present invention relates generally to a semiconductor rectifier and, more particularly, to a thyristor which is controlled by radiation, such as a light beam.
It is well known that a thyristor valve is preferably utilized as a rectifier which controls power transmission when used in a high power transmission system such as a DC power transmission system or the like. In this regard, persons skilled in the art have urged that the ordinary electrical trigger type thyristor, which controls the start time of conduction by an electrical triggering signal supplied to the gate electrode thereof, should be replaced by a light trigger type thyristor (photo-thyristor), such as a thyristor valve, particularly in the case of a power converting apparatus of the above-mentioned power transmission system. Such a replacement is needed, because the use of a light trigger thyristor in a high-voltage power converting apparatus, in which the capacitance and voltage have tended to increase over the years, allows countermeasures for inductive interference and electrical insulation between a main circuit and a control circuit to be easily taken; thus, it can be expected to realize an extremely small and lightweight power converting apparatus.
However, since the light energy to be used in triggering the photo-thyristor is weaker than the electrical triggering energy, it is necessary to increase the photo-sensitivity of the photo-thyristor itself (e.g., in the order of approximately several ten times larger than the gate sensitivity of the electrical trigger type thyristor) in order to effectively control the operation of the photo-thyristor. An increase in the light gate sensitivity of the photo-thyristor causes the noise-resisting property to become worse, since malfunction will easily occur against the voltage noise having a steep leading waveform to be applied from the main circuit side such as a lightning surge or the like to the photo-thyristor. The critical off-voltage build-up rate at which a malfunction of the thyristor will not occur, even if an overvoltage such as the above-mentioned voltage noise or the like is applied thereto, is called the "dv/dt resistive amount". It has been known that it is possible to improve the dv/dt resistive amount without making the photo-sensitivity of a thyristor worse by reducing the photo-sensitive area of the photo-thyristor and reducing the interference current to be caused in this area. However, reduction of the photo-sensitive area causes the conductive range at the initial time of turn-on in an operating characteristic of the photo-thyristor to decrease; consequently, the resistive amount (which is well known as "di/dt resistive amount" to those skilled in the art) of the photo-thyristor against the on-state current having a steep leading waveform that will be caused at the initial time of turn-on will decrease. Therefore, it is one of the important technical subjects at present to develop a photo-thyristor having a high photo-sensitivity without making principal thyristor characteristics of the dv/dt and di/dt resistive amounts worse.